Carbon dioxide laser employing multiple gases including oxygen and water vapor

ABSTRACT

There is disclosed a laser providing emission of coherent radiation near 10 microns in the far infrared and utilizing transitions between vibrational-rotational levels in carbon dioxide. Also disclosed are beneficial effects from addition of oxygen and water vapor to various forms of such a laser.

United States Patent 1 Patel Mar. 27, 1973 I73] Assignee: Bell 1 CARBON DIOXIDE LASER EMPLOYING MULTIPLE GASES INCLUDING OXYGEN AND WATER VAPOR [75] Inventor: Chandra K. Patel, Chatham, NJ.

Telephone Laboratories, In-

corporated, Murray Hill, Berkeley Heights, NJ.

[22] Filed: Nov. 30, 1970 211 App]. No.: 93,565

Related US. Application Data [60] Division of Ser. No. 814,510, March 28, 1969, Pat. No. 3,596,202, which is a continuation-in-part of Ser. Nos. 409,682, Nov. 9, 1964, abandoned, and Ser. No. 474,546, July 26, 1965, abandoned, and Ser. No. 495,844, Oct. 14, 1965, abandoned.

52 user ..331/94.so,252/372 [51] 1m.c| ..H01s3/00 [58] FieldofSearch ..331/94.5;252 372 c0 OLANT EXHAUST MIXING REGION 2 SOURCE C02 SOURCE 2 O2 VAPOR SOURCE SOURCE SOURCE or I l AUXILIAR! GASES PUMP EXHAUST l [56] References Cited OTHER PUBLICATIONS Howe: Effect of Foreign Gases on the Carbon Dioxide Laser: R-Branch Transitions, Applied Physics Letters, Vol.7, pp. 21-22, July 1, 1965.

Primary ExaminerRonald L. Wibert Assistant ExaminerEdward S. Bauer Attorney-R. J. Guenther et al.

[5 7] ABSTRACT There is disclosed a laser providing emission of coherent radiation near 10 microns in the far infrared and utilizing transitions between vibrational-rotational levels in carbon dioxide. Also disclosed are beneficial effects from addition of oxygen and water vapor to various forms of such a laser.

- 8 Claims, 7 Drawing Figures COOLANT SOURCE MIXING REGION CO2 SOU RCE 2 02' SOURCE SOURCE SOURCE OF AUX|LIA RY GASES PATENTEDHARZ? 197a SHEET 20F 7 sc gmm muzmuwwmo u 12o oooom Gm/3 ooo ooom

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SHEET 7 OF 7 FIG. 7

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EXHAUST INCLUDING PUMP PUMP OR EXPANSlON M4 M02 ZLE HYDROCARBON AND AIR COMBUSTION CHAMBER CARBON DIOXIDE LASER EMPLOYING MULTIPLE GASES INCLUDING OXYGEN AND WATER VAPOR RELATED APPLICATIONS 409,682, filed Nov. 9, 1964; Ser. No. 474,546, filed July 26, 1965; and Ser. No. 495,844, field Oct. 14,

1965, the latter three of which applications are now abandoned. The other divisional application is Ser. No. 93,564.

BACKGROUND OF THE INVENTION This invention relates to gas lasers. A gas laser is a device which includes a gaseous active medium capable of the stimulated emission of coherent radiation and means for pumping the medium to populate selectively an energy level which is optically connected to a lower energy level, whereby there is established a population inversion with respect to such two levels. In a laser oscillator, the overall gain must exceed the losses, including the fractional power output. To achieve oscillations, it is important to enclose the active medium in an optical resonator, from which a portion of the coherent radiation can be extracted. In a laser amplifier an optical resonator is not necessary; but it is important to make provision for introducing a beam to be amplified and for extracting the amplified beam.

The incorporation of the foregoing components into an optical device was proposed by A. L. Schawlow et al. in their US. Pat. No. 2,929,922, issued Mar. 22, 1960.

The stimulated radiation has several properties which are usually referred to by calling it coherent radiation. The first is a very narrow frequency spectrum, as well as a desired phase relationship. This aspect of coherency is sometimes termed temporal coherency. The second typical property of the coherent light is its capability for being confined in a narrow beam, which property is sometimes termed spatial coherency. One attractive aspect of gas lasers is the high degree of spatial and temporal coherency that can be obtained.

An attractive aspect of gas lasers consists of the variety of different processes which may be used in the formation and destruction of excited states, in order to obtain population inversion between a pair of optically gas systems where population inversion was accom-' plished by optical pumping, electron impact, excitation transfer in nonelastic atom-atom collisions, and by molecular dissociation in atom-molecule collisions. In

radiation is stimulated; and there is avoided undesired Y addition, in my prior application filed Mar. 15, 1963,

now US. Pat. No. 3,411,105, pumping of gaseous atoms by electron impact is disclosed in discharges in which molecules have been dissociated.

An important deficiency of gas lasers hitherto has been their very low efficiency, typically small fractions of one percent lpercent). One interesting approach to this' problem is that suggested by J. C. Polanyi, Proposal for an Infrared Laser Dependent upon Vibrational Excitation, Journal of Chemical Physics, Vol. 34, page 347 (1 961). Polanyi points out that the radiative lifetimes for vibrationally excited states are several orders of magnitude greater than for electronic states, thus facilitating attainment of relatively high concentrations of selectively excited vibrational species. In addition, vibrational excitations can typically be very efficiently produced. Nevertheless, the specific gases, sodium iodide and hydrogen chloride, suggested by Polanyi are not suitable choices for providing outputs of coherent radiation.

An object of the present invention is a gas laser of improved efficiency and power output.

SUMMARY OF THE INVENTION I have discovered that the emission of coherent radiation near 10 microns in the far infrared may be efficiently obtained from transitions between vibrationalrotational levels in carbon dioxide. I have also discovered beneficial effects upon efficiency and power output from addition of oxygen and water vapor. The beneficial effects of the addition of helium are disclosed in my above-cited copending divisional application.

One specifically described species of the invention involves a two-gas system, one of which serves as the exciting gas to help establish a population inversion, and the other as the lasing gas from which the desired excitation of the lasing gas by avoiding any treatment of the lasing gas which gives rise to such undesired excitation. Specifically, the lasing gas is not itselfsubjected to its discharge, in contradistinction with the usual practice in prior art forms of gas lasers.

Nevertheless, as disclosed in my earlier article Interpretation of CO Optical Maser Experiments," Physical Review Letters, Vol. 12, page 588 (May 25, 1964), an earlier embodiment of my invention, for establishing the population inversion, employed the electron impact excitation of carbon dioxide directly. Such embodiments are attractive for many applications because of their simplicity.

Carbon dioxide lasers according to my invention may also be pumped by chemical reactions generating carbon dioxide and vibrationally excited nitrogen.

In this first-mentioned embodiment of the invention in which nitrogen serves as the exciting gas and carbon dioxide as the lasing gas, the nitrogen is excited in a separate chamber and thereafter made to flow therefrom into an interaction chamber for mixing there with the carbon dioxide, which is introduced in an unexcited state. A continuous flow of both gases is employed for a continuous operation. The carbon dioxide becomes excited in the interaction chamber by colli-' sion with the excited nitrogen in a highly selective fashion. The interaction chamber is positioned within an optical resonator which provides the regeneration,

to facilitate achieving stimulated emission from the excited carbon dioxide.

The excited nitrogen, which is the exciting medium, includes an excited level which has a relatively long lifetime, typically longer than 0.1 second, and which is not readily destroyed by wall collisions. In addition, as

pointed out in the article by Morgan and Schiff, The Study of vibrationally Excited N with the Aid of an Isothermal Calorimeter, Canadian Journal of Chemistry, Vol. 41, page 903 at 909 and 910 (1963), this nitrogen level is closely matched in energy to a carbon dioxide vibrational level, which I had found is the upper laser level of the 10.6 micron transition employed in my laser.

With respect to the beneficial effects of other gases, I have discovered that the addition of oxygen to the gaseous medium of a carbon dioxide vibrational-rotational transition laser makes feasible the use of higher total gas pressure in the lasing gas, which in turn results in increased efficiency and increased power output. Further, I have discovered that the addition of water vapor to the laser also increases the efficiency and power output. By such additions, continuous-wave stimulated radiation was obtained at 10.6 microns at a power level of about 16 watts with an efficiency of more than four percent (4percent).

BRIEF DESCRIPTION OF THE DRAWING The invention will be further described in the following more detailed description, taken in conjunction with the accompanying drawings, in which:

FIG. 1 shows schematically a partially cut-away view of an illustrative embodiment of the invention;

FIG. 2 is a partial energy-level diagram of the N CO system, which will be useful in describing the invention;

FIG. 3 shows schematically a modification of the embodiment of FIG. 1;

FIG. 4 shows schematically and block diagrammatically a partially cut-away view of another embodiment of the invention employing oxygen and water vapor;

FIG. 5 shows an energy-level diagram pertinent to the embodiment of FIG. 4;

FIG. 6 shows a modification of the embodiment of FIG. 4; and

FIG. 7 shows another modification of the embodiment of FIG. 1.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS With reference now to the drawing, the laser shown in FIG. 1 includes a first section where the nitrogen which serves as the exciting gas is treated for the production of molecules in the vibrational level of the ground electronic state. In the embodiment depicted, this involves subjection of the nitrogen to a d-c discharge where electron-ion recombinations, atomatom recombinations and excitation indirectly by electron impact on nitrogen molecules as well as cascades provide the desired excitation of the nitrogen molecules. In particular, nitrogen, advantageously of a purity better than 99.99percent, is introduced into the discharge region via an inlet port I] for continuous flow, for example, at a linear flow rate of the order of 500 centimeters per second. The discharge is confined within the multisection glass tube 12. In the design being described specifically, this tube has an inner diameter of about millimeters. The discharge region in this tube is defined by the cathode l4 and the pair of anodes 15. The use of two spaced anodes permits elongation of the discharge path without a corresponding increase in the d-c voltage required. A d-c voltage source (not shown) supplies the power necessary. For the design being described involving a separation of about 00 centimeters between the cathode and each anode, 2,000 volts with a current flow of 9 milliamperes proved satisfactory. Such voltages and currents provide a discharge that is substantially cooler than the discharge employed in my above-cited U.S. Pat. No. 3,411,105, and produces much less molecular dissociation in the discharge. 4

The now excited nitrogen is made to flow out of the discharge region into the interaction region, into which is also introduced unexcited carbon dioxide, also advantageously of high purity, by way of inlet port 17 and distribution tube 18. The interaction region is enclosed within a glass tube 20. In the design being described, this tube had an inner diameter of about 25 millimeters. To improve the mixing of the exciting nitrogen and the lasing carbon dioxide, the nitrogen and carbon dioxide are each introduced into the interaction region at a number of separate inlets 21, spaced apart along the interaction region as shown. To keep small the amount of atomic nitrogen introduced into the interaction region, and thereby to improve the efficiency of the interacting, the distance between the discharge region and the point at which nitrogen enters the interaction region advantageously is made in each instance at least 6 centimeters to provide time for substantial completion of all the atomic and electron-ion recombination in the nitrogen before mixing with the carbon dioxide. In the design discussed, the carbon dioxide and nitrogen flow rates were adjusted to be nearly equal with a total gas pressure of 0.8 Torr in the interaction region.

The spent gases are withdrawn from the interaction region by way of the exhaust port 30 under the action of a pump 31 which controls the flow. Advantageously, as shown, the mixture evacuated is supplied to apparatus shown schematically for separating the two components and after separation returning the separate components to the appropriate inlet ports of the system. The separation apparatus has not been shown in detail as the techniques for such separation are known. Typically, separation would involve passing the mixture through a cold trap for solidification of the carbon dioxide.

To achieve oscillation, it is important to enclose the interaction region in an optical resonator. In the specific design being discussed, the resonator was formed by a pair of near confocal concave mirrors 33, 34 spaced apart about 1.4 meters and supported by suitable mirror mounts 35, 36, respectively. The mirrors are illustratively pressed polycrystalline zinc sulfide coated to be opaque with vacuum-deposited gold, and energy was coupled out from the resonator through a 0.5 millimeter aperture in the center of mirror 33.

If the laser be intended for use as a straight-through amplifier, the resonator becomes unnecessary and the mirrors can be eliminated. In some instances, it may be preferred to locate the mirrors outside the interaction region, in which case Brewster-angle windows of material transmissive at 10.6 microns, for example, pressed polycrystalline zinc sulfide, barium fluoride or potassium chloride, can be used to confine the interaction region.

A discharge in low pressure nitrogen results in very efficient production of nitrogen molecules in the vibrational levels of their ground electronic state. Since nitrogen has a zero permanent dipole moment,-such molecules cannot decay to the v 0 vibrational level through electric dipole radiation. Thus, the effective lifetimes of these states are governed prim arily by deactivation through collisions with other molecules and walls, and accordingly can be high, for example, several seconds.

In FIG. 2, there is shown the vibrational level v l for nitrogen with respect to the ground state of nitrogen. The other vibrational levels have been omitted for simplicity as unimportant. There have also been shown the vibrational energy levels of carbon dioxide of interest, with respect to its ground state (000). It can be seen that N (v 1) at 2330.7 cm is in very close coincidence with the (00l) vibrational level of CO at 2349.16 cm. Thus, a collision of the second kind can have a large cross section because of the near-perfect coincidence. The lower laser level (0) on the other hand is removed from N (v l by more than 900 cm and hence the cross section for exciting the CO ground state molecules to the lower laser level will be very much smaller. Moreover, the excitation of CO (000 molecules to level (100) involves a reaction which includes transitions which are optically forbidden. Thus, when the excited nitrogen molecules are allowed to mix with the carbon dioxide molecules, a selective excitation of CO ground state molecules to the (00l) level takes place. Moreover, since the lifetimes of the (00l) levels are longer than the lifetimes of' the (100) levels, the required conditions for laser action on the (00l) (100) transitions are satisfied. The particles thereafter relax from the (100) level to the ground state in cascade fashion as depicted schematically. The decay processes which are most important are the oneswhich involve collisions with other gas particles.

Laser oscillations were obtained at a number of wavelengths involving P-branch rotational transitions from P(14) to P(26). The strongest transition, corresponding to P(20), occurs at 10.5915 microns, and continuous wave power in excess of l milliwatt was readily obtained with less than 20 watts input power. As disclosed in my above-cited article of May 25, 1964, laser oscillation was also obtained on vibrational-rotational transitions of the carbon dioxide at 9.6 microns.

There are a large number of reactions through which one can produce various diatomic molecules in vibrationally excited levels and hence the selective excitation through vibrational energy transfer is applicable for obtaining laser action on vibrational-rotational transitions of polyatomic molecules in other mixed systems. Molecules in vibrationally excited levels are of special importance for both the exciting and the lasing gases because of the longer lifetimes available with such molecules. Another requirement on the exciting gas is that its excited molecules not be easily destroyed by wall collisions, making it feasible for transport of these molecules from the discharge region to the interaction region without excessive destruction. In some instances, it may be advantageous to treat the walls to minimize their effect on the lifetimes of incident excited particles. Also, it is advantageous that the molecule exhibit no permanent dipole moment so as to avoid radiative transitions between vibrational levels. This ordinarily means that the molecule of the exciting gas should be composed of like atoms. Additionally, of course, there is required the close coincidence between the vibrational level of the exciting gas and an upper level of the lasing gas, which level is optically connected to a lower level.

The specified conditions are also met, for example, in an N N 0 system and an O CS system (the first gas being the exciting gas and the second the lasing gas). In particular, the apparatus described has been used to generate oscillations in the N -N O system at 10.8422 microns and other wavelengths.

It is a further advantage of lasers of the kind described that due to the absence of any discharge in the interaction region, the usual inverse relation of optical gain with diameter of the interaction region is typically not applicable here. Thus, the diameter of the interaction chamber can be increased with little adverse effect on the optical gain. Rather, because an increase in diameter can be translated into an increase in number of excited molecules, the optical power output can be increased by increasing the diameter of the in teraction chamber.

Various other techniques are feasible for the excitation of the exciting molecules. Such modifications include the use of a radio frequency discharge rather than a d-c discharge and, most simply, heating the exciting gas to populate the desired v 1 level. For exampie, with nitrogen at 1440 I(., the population corresponding to the Boltzmann distribution in the v l vibrational level is about ten percent (10%) of the total nitrogen population, which is more than adequate for achieving lasing action with carbon dioxide. Such heating can be achieved simply by flowing the nitrogen past a heating element before introduction into the interaction region. In FIG. 3 such heating is illustratively provided by a heating element 44 of known type disposed within the region defined by tube 12. Heating element 44 may, for example, be energized from an electrical current source 45. Additionally, chemical reactions, for example, as described in the paper entitled Vibrational Disequilibrium in Reactions Between Atoms and Molecules, Canadian Journal of Chemistry, Vol. 38, page 1769 et seq. (1960), can be employed to populate vibrational levels.

A modification of the embodiment of FIG. 1 excited by a chemical reaction of the type disclosed in the justcited article is shown in FIG. 7. A hydrocarbon is burned in air in combustion chamber 53, which for example could include a common Bunsen burner, and caused to flow through ducts 112 and 1-7 by a pump or, advantageously, an expansion nozzle 54. It is known that hydrocarbon-air flames reach a temperature of about 2500-3000 C. The flame products include CO and N both of which are vibrationally excited because of the high temperature of the flame. Expansion nozzles are well-known means for causing heated gases to flow and reduce the molecular and rotational temperature of molecules at the same time but leave the vibrational temperature unaffected which is a desired condition for laser action on molecular vibrational-rotational transitions. The combustion produces adequate amounts of both carbon dioxide and vibrationallyexcited nitrogen. The spent gases are then exhausted to the atmosphere.

In embodiments of my invention employing electron impact excitation of carbon dioxide, the parameters of the discharge in carbon dioxide are similar to those disclosed above for the discharge in nitrogen. That is, the voltages per unit length and the currents are such that the degree of molecular dissociation produced is not great enough to interfere with the generation and coupling out of radiation from the vibrational-rotational transitions near 10 microns. More especially, it is important to maintain a desired concentration of undissociated molecules of carbon dioxide.

In order to improve the power output and efficiency of the CO lasers described above, it is important to (a) increase the depopulation rate for the lower laser level (00! or 020) or (b) increase the rate at which CO molecules are excited to the upper laser level (001).

I have discovered that oxygen and water vapor are additives which are effective to increase efficiency and power output in a molecular carbon dioxide laser, as illustrated in FIG. 4.

In FIG. 4, a laser operated according to the present invention included an interaction region comprising a tube 20 in which the lasing action was obtained, apparatus for furnishing therein a gas mixture containing additives as I have discovered, and apparatus for exciting the gas mixture to enable the stimulated emission of radiation therefrom.

Illustratively, the gas mixture was furnished by a continuous flow thereof from suitable sources 41-44 of carbon dioxide, nitrogen, oxygen and water vapor,

respectively. In several of the experiments, portions of the nitrogen, oxygen and water vapor were provided by the inflow of moist room air. For these experiments, sources 42, 43 and 44 may be more conveniently described as a source 46 of auxiliary gases, even though a substantial supplementary flow of nitrogen gas was provided from a tank of gaseous nitrogen in some of these experiments.

In any event, the gases were flowed through mixing regions 32 and inlet apparatus 12 into the interaction region; and the spent gases were removed from the interaction region to the exhaust 40 by a pump 31.

The gas mixture was excited by electrical discharge apparatus disposed, for convenience, in one branch of the inlet apparatus 12. Specifically, the discharge apparatus included the oxide-coated cathodes 14 disposed in bulbous appendages 19 of apparatus 12 near opposite ends of tube 20, the anode 15 disposed in the bulbous appendage 17 of tube 20, the d-c voltage sources 13 with positive terminals connected to anode 15 through current-limiting resistors 16 and negative terminals connected to cathodes l4, and the heater current sources 22 connected across cathodes 14. Because the anode 15 is disposed in the interaction region of tube 20, the discharge runs directly through the interaction region, in which the lasing action is obtained.

To increase the so-called population inversion between the upper and lower laser levels, i.e., the energy states associated with the stimulated radiation, it is desirable to increase the difference between the socalled rotational temperature, the molecular temperature and the vibrational temperature of the carbon dioxide. At the normal pressures of operation of carbon dioxide and other molecular lasers of this type, the rotational temperature and molecular temperature are very nearly equal. Thus we now want to obtain a high vibrational temperature but a low rotational temperature. To this end, the tube 20 was surrounded by a jacket 21 through which was flowed a coolant from source 26 to exhaust 27. Tap water at 15 C. was used in the experiments described in detail herein; but coolants capable of depressing the rotational temperature of the gas even further below room temperature were still more advantageous. The coolant determined the wall temperature of tube 20 and thus depressed the rotational temperature of the carbon dioxide. It appeared that the vibrational temperature was not substantially affected by the coolant.

To achieve oscillation, it is important to include the interaction region in an optical resonator. In the specific design being discussed, the resonator was formed by a pair of mirrors 33 and 34, the latter concave with l I meters radius of curvature and the former convex with 10 meters radius of curvature in order to enlarge the mode volume to use the entire gas mixture within the interaction region for laser action. The mirrors were spaced at 240 centimeters. The mirrors were supported by suitable mirror mounts 35 and 36, respectively, and were coated to be opaque with vacuumdeposited gold. Energy was coupled out from the resonator through a 12.5 millimeter aperture in the center of concave mirror 34.

If the laser be intended for use as a straight-through amplifier, the resonator becomes unnecessary; and the mirrors can be eliminated. In some instances, it may be preferred to locate the mirrors outside the interaction region, in which case Brewster-angle windows (for example, of crystalline zinc sulfide, barium difluoride or potassium chloride) can be used to confine the interaction region. Such a modification of the embodiment of FIG. 4 including windows 47 and 48 of such materials is shown in FIG. 6. It should be apparent that the substrates of the partially transmissive mirrors advantageously may be of such materials.

The tube 20 in which the multiple gases interacted in the embodiment of FIG. 4 was a glass tube, although it could be some other rigid tubular structure, such as a tube of nonconducting plastic or quartz. The glass tube 20 extended the distance between mirrors 33 and 34 and had an inside diameter of 25 millimeters. The effective interaction region was observed to extend between the points at which apparatus 12 enters tube .20 and was about 2 meters long.

Cathodes 14 were oxide-coated cathodes having 220 nickel bases and containing barium carbonate and strontium carbonate in the respective proportions percent 45 percent. The nickel bases inherently included such activator impurities as Si, Ti, Al and Mg in very small amounts.

Laser oscillation was obtained at a numberof wavelengths involving P-branch rotational transitions P( 14) to P(26), primarily P(l8), P(20) and P(22). The strongest transition, corresponding to P(20), occurred at 10.5915 microns.

For each gas mixture, the laser power output was measured by a calibrated thermopile disposed beyond the aperture of concave mirror 34. It should be understood that the output stimulated radiation has a variety of uses and that the calibrated thermopile could illustratively be replaced by a desired utilization apparatus 25, which could, for example, be an electrooptic modulator responsive to an information signal. The invention is particularly useful for communication, inasmuch as the atmosphere has relatively low attenuation at 10.6].L. The wavelength range nearto 10.6 is commonly called an atmospheric window.

One specific example of the operation of my invention was obtained by mixing carbon dioxide having a pressure of 0.4 Torr with room air having a partial pressure of about 1.0 Torr made up essentially of about 0.7 Torr N 0.3 Torr O and 0.04 Torr H and with additional nitrogen gas having a partial pressure of about 1.0 Torr. Tap water was flowed through jacket 21 at C. With each of sources 13 providing 50 milliamperes of current at 4,000 volts, i.e., about 400 watts of power,'a continuous-wave power output of 16.2 watts from the laser was measured by the calibrated thermopile. Other experiments, which involved flowing alcohol through jacket 21 at temperatures and rates that provided rotational temperatures down to 40 C. in the gas mixture within tube 20, provided improved efficiency and power in direct relation to the amount by which the rotational temperature of the gas was depressed below the vibrational temperature of the gas I mixture.

When the input air was dried with calcium chloride to remove the water vapor, the coolant being tap water atl 5 C., a power output of 12 watts was obtained.

For purposes of comparison, measurements were made for a nearly optimum mixture of CO 'and N with no other additives. With the partial pressure of nitrogen approximately equal to 0.4 Torr and the partial pressure of carbon dioxide approximately equal to 0.4 Torr, the coolant being tap water at 15 C., a power output of 4.5 watts was obtained.

It should be particularly noted that the nitrogen partial pressure was much lower in the mixture without additives than in the previously described mixtures with additives and, if raised beyond about 1.0 Torr without additives, tended to reduce the power output. For the system without additives, the power output and efficiency were observed to be fairly constant for nitrogen pressures between 0.4 and 1.0 Torr. However, the additives oxygen and water vapor permitted the nitrogen partial pressure to be increased further with a corresponding increase in power output.

It appears that the oxygen partial pressure within the interaction region should be greater than 0.1 Torr for an appreciable effect in increasing the efficiency of the laser, but less than 0.5 Torr. For an appreciable additional effect from the water vapor, its partial pressure within the interaction region should be greater than 0.01 Torr; but less than 0.1 Torr. The carbon dioxide pressure should be greater than 0.1 Torr and less than 1.0 Torr; and the nitrogen pressure should be greater than 0.1 Torr and less than 10.0 Torr.

An alternative embodiment of my invention would employ an N N O gas mixture with O and H 0 additives, the excitation of nitrous oxide for lasing action being carried out in substantially the same way as the excitation of carbon dioxide for lasing action.

Further experiments relating to my invention are described in my articles in the Applied Physics Letters, July 1965, and in the Proceeding of the Physics of Quantum Electronics Conference, San Juan, Puerto Rico, June 1965.

I suggest, without wishing to limit my invention hereby, that a variety of theoretical considerations may be applicable, in varying degrees, to these observed results.

For the first such consideration, reference is made to FIG. 5, which shows the vibrational level v l and two other higher excited levels for nitrogen with respect to the ground state of nitrogen. There have also been shown the vibrational levels of carbon dioxide of interest, with respect to its ground state (000'). It can be seen that the N (v 1) level at 2330.7 cmis in very close coincidence with the (001) vibrational level of CO at 2349.16 cm". Thus, a collision of the second kind can have a large cross section because of the nearperfect coincidence. The lower laser level (100) on the other hand is removed from N (v 1) by more than 900 cm and hence the cross section for exciting the CO ground state molecules to the lower laserlevel will be very much smaller. Moreover, the excitation of CO (000) molecules to level (100) involves a reaction which includes transitions which are optically forbidden. Thus, when the excited nitrogen molecules transfer energy to the carbon dioxide molecules, a selective excitation of CO ground state molecules to 'the (00l) levels takes place. Moreover, since the lifetimes of the (001) levels are longer than the lifetimes of the (100) levels, the required conditions i for laser action on the (001)(l00) transitions are satisfied. The shorter lifetime of the lower level is associated with the fact that the population of the lower level relaxes to the ground state through collisions in cascade fashion as depicted schematically.

An important point in the process just described is the efficient excitation of the nitrogen (v 1) vibrational level. It is known that it is relatively difficult to excite this level directly by electron impact. Accordingly, it is consistent with the observed results that the electron discharge in tube 20 excites, by impact, the higher excited levels in the nitrogen, as indicated. The population density of these short-lived excited levels then decays through spontaneous radiative transitions to populate the v 1 level, as indicated.

Vibrational excitation of N by electron impact also occurs through a compound state of N existing at about 2.3 ev (frequency, about 18,400 cm) while the vibrational excitation of CO by electron impact occurs through a compound state of CO existing at about 1 ev (frequency, about 8000 cm"). Thus, it is important to have as many electrons as possible which have their kinetic energies ranging from 1 2.3 ev.

Lower efficiencies would tend to occur in such a laser Without additives for the following reason. In a mixture of CO and N, without additives the electron density in the discharge as a function of electron energy is as indicated by curve 51. The peak of this curve lies between the v 1 level and the higher excited levels, but substantially lower than about 2.3 ev; and the energy expended by the applied electric field in accelerating these median-energy electrons is wasted, since they cannot excite any of the nitrogen levels and may pass out of the interaction region without being of use in the excitation of nitrogen or the upper laser level of the carbon dioxide.

A possible distribution of electron energies in the discharge with oxygen is shown by curve 52. Since the vertical axis of these curves represents energy and the horizontal axis represents relative numbers of electrons, it is seen that a substantial number of electrons would be shifted to higher energies.

Thus, it is possible that, with the addition of oxygen, the median electron energy, or electron temperature, as it is sometimes called, will move to higher energies until the excitation of the compound state of nitrogen establishes an equilibrium, as indicated by the dashed curve 52 of FIG. 5. Moreover, this tendency of oxygen, or an electronegative element, to raise the electron temperature would account for the fact that the total nitrogen pressure can be usefully increased, maintaining the ratio of electron temperature to gas pressure at a selected level, inasmuch as a sufficient number of high-energy electrons are now available to excite the compound state of substantially all of the nitrogen in spite of the increased number of electron-molecule collisions that tend to reduce electron energy. The horizontal displacement or bulge in each curve indicates relative numbers of electrons at the different energies. The greater numbers of electrons at energies exceeding or equal to the energy of the compound state of nitrogen are much more effective in populating that nitrogen energy level than are the lower energy electrons. The excited compound state of N then decays efficiently to the nitrogen energy level which is effective in pumping carbon dioxide molecules to enable the 10.6 micron stimulated emission of radiation.

It is now understood that one beneficial role of water vapor in lasers according to my invention resides in its ability to relax the excitation of molecules having the vibrational energy of the lower laser level. As is shown in my above-cited copending divisional application, Ser. No. 93,564, helium is also important for raising the average electron energy in the discharge, in a manner similar to that shown for oxygen in P16. 4.

In all cases, it is understood that the above-described arrangements are illustrative of a small number of the many possible embodiments that can represent applications of the principles of the invention. It can be appreciated that many of the design parameters are interrelated so that in most instances the optimum mixture for a devised tube design is best determined experimentally consistent with the principles outlined. Numerous and other arrangements can readily be devised in accordance with these principles by those skilled in the art without departing from the spirit and scope of the invention. In particular, other arrangements can be used for exciting the gas discharge, such as a radio frequency discharge.

ln all of the foregoing embodiments, the gases designated to be present in each of the lasers is present in an amount at least two orders of magnitude greater than trace amounts. 1n lasers, such as the well-known helium-neon laser, trace amounts are considered to be amounts less than about parts per million by partial pressure.

1 claim: 1. An infrared laser, comprising means containing a mixture of gases and permitting the abstraction of radiation from said gases, said mixture comprising an active gas including carbon dioxide, which gas is capable of lasing action between a pair of vibrational-rotational energy levels thereof via radiation at about 10 microns, an auxiliary gas having an excitable level that couples energy to the upper of said pair of levels to promote said lasing action, an additive comprising molecular oxygen having a partial pressure greater than 0.1 Torr, and means for applying electromagnetic wave energy to said mixture to excite said excitable state of said auxiliary gas.

2. A laser according to claim 1 in which the active gas consists essentially of carbon dioxide, and the auxiliary gas includes molecular nitrogen.

3. A laser according to claim 1 including within the mixture an additional additive consisting essentially of water vapor having a partial pressure greater than 0.01 Torr.

4. An infrared laser, comprising elongated means containing a mixture of gases and including an optical resonator permitting the abstraction of coherent radiation at about 10 microns along the axis of elongation of said means, said mixture including water vapor having a partial pressure greater than 0.01 Torr, oxygen, molecular nitrogen and carbon dioxide, and means for creating an electric discharge in said gas mixture to excite nitrogen molecules in said mixture to a vibrational energy level with a large cross-section for excitation of the upper level of a pair of vibrational rotational energy levels of said carbon dioxide, downward transitions between said pair of levels producing said radiation at about 10 microns, said water vapor and oxygen providing increased efficiency of conversion of the energy of said discharge to said radiation.

5. A laser according to claim 4 in which the partial pressure of the water vapor in said mixture is greater than 0.01 Torr and less than 0.1 Torr.

6. An infrared laser of the type comprising means through which a mixture of gases may be flowed, means for producing an electric discharge in said mixture within said means to enable the stimulated emission of radiation at about 10 microns, means for resonating and abstracting said radiation, said mixture including a first gas consisting essentially of carbon dioxide, a second gas including nitrogen, a third gas including water vapor having a partial pressure greater than 0.01 Torr, and a fourth gas including molecular oxygen having a partial pressure greater than 0.] Torr.

7. A laser according to claim 6 in which the first gas has a partial pressure greater than 0.1 Torr and less than 1.0 Torr, the nitrogen has a partial pressure greater than 0.1 Torr and less than 10.0 Torr, the water vapor has a partial pressure in the range from 0.01 Torr to 0.1 Torr, and the oxygen has a partial pressure in the range from 0.1 to 0.5 Torr.

8. A laser according to claim 7 in which the nitrogen has a partial pressure greater than 1.0 Torr. 

2. A laser according to claim 1 in which the active gas consists essentially of carbon dioxide, and the auxiliary gas includes molecular nitrogen.
 3. A laser according to claim 1 including within the mixture an additional addiTive consisting essentially of water vapor having a partial pressure greater than 0.01 Torr.
 4. An infrared laser, comprising elongated means containing a mixture of gases and including an optical resonator permitting the abstraction of coherent radiation at about 10 microns along the axis of elongation of said means, said mixture including water vapor having a partial pressure greater than 0.01 Torr, oxygen, molecular nitrogen and carbon dioxide, and means for creating an electric discharge in said gas mixture to excite nitrogen molecules in said mixture to a vibrational energy level with a large cross-section for excitation of the upper level of a pair of vibrational rotational energy levels of said carbon dioxide, downward transitions between said pair of levels producing said radiation at about 10 microns, said water vapor and oxygen providing increased efficiency of conversion of the energy of said discharge to said radiation.
 5. A laser according to claim 4 in which the partial pressure of the water vapor in said mixture is greater than 0.01 Torr and less than 0.1 Torr.
 6. An infrared laser of the type comprising means through which a mixture of gases may be flowed, means for producing an electric discharge in said mixture within said means to enable the stimulated emission of radiation at about 10 microns, means for resonating and abstracting said radiation, said mixture including a first gas consisting essentially of carbon dioxide, a second gas including nitrogen, a third gas including water vapor having a partial pressure greater than 0.01 Torr, and a fourth gas including molecular oxygen having a partial pressure greater than 0.1 Torr.
 7. A laser according to claim 6 in which the first gas has a partial pressure greater than 0.1 Torr and less than 1.0 Torr, the nitrogen has a partial pressure greater than 0.1 Torr and less than 10.0 Torr, the water vapor has a partial pressure in the range from 0.01 Torr to 0.1 Torr, and the oxygen has a partial pressure in the range from 0.1 to 0.5 Torr.
 8. A laser according to claim 7 in which the nitrogen has a partial pressure greater than 1.0 Torr. 